RENO, Nev. — Conservationists sued the U.S. Fish and Wildlife Service on Thursday seeking protection for a rare butterfly they say is threatened by off-road vehicles at one of the largest sand dunes in the West.

Environmentalists want the agency to declare the Sand Mountain blue butterfly an endangered species because, they say, its habitat is being destroyed at the only place it is known to live — the Sand Mountain Recreation Area in western Nevada.

The Bureau of Land Management controls activities at the dune, which is 600 feet tall and stretches for two miles. It attracts an estimated 50,000 off-roaders annually on motorcycles, dune buggies and all-terrain vehicles.

The lawsuit, filed in federal court in Sacramento, Calif., accuses the agency of violating the Endangered Species Act by failing to respond to petitions since April 2004 seeking federal protection for the butterfly. The act requires the government to provide a preliminary response to such petitions within 90 days and often again within a year.

Plaintiffs accuse the Bureau of Land Management of pandering to off-roaders.

“It’s a stall tactic coming out of hostile politics in Washington,” said Daniel Patterson, a desert ecologist for Arizona’s Center for Biological Diversity, which is suing along with the Nevada Outdoor Recreation Association and Public Employees for Environmental Responsibility.

Bob Williams, Nevada supervisor for the Fish and Wildlife Service, blamed lack of money for the agency’s inaction.

“We only get a certain amount of money a year to do listing findings,” Williams said. Most of it was spent on sage grouse and pygmy rabbit reviews in the past two years, he said.

The Fish and Wildlife Service hopes to spend about $35,000 on a 90-day review — probably beginning in June — to determine whether there is enough evidence to warrant a full, yearlong review of the butterfly, Williams said.

“We do know the butterfly is there, and we haven’t been able to find it at any other location,” he said.

The butterfly depends on a unique shrub, the Kearney buckwheat, which covers about 1,000 of the recreation area’s 4,795 acres. It is found nowhere else.

The Fish and Wildlife Service and the Bureau of Land Management have been working with local groups to save the butterfly without necessarily listing it as endangered. Tentative plans call for more signs designating approved travel routes, fencing to protect the habitat and additional law enforcement.

Many off-roaders oppose a federal listing.

“If it did become listed, no telling what type of restrictions they could do out there,” said Richard Hilton of Reno, a board member of the Friends of Sand Mountain, a group composed primarily of off-road enthusiasts.